The disclosure of Japanese Patent Application No. 2000-388506 filed on Dec. 21, 2000, including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to brake control apparatus and methods for a vehicle, such as an automobile, and more particularly to brake control apparatus and methods for controlling a pressure in a wheel cylinder of each wheel using a high-pressure hydraulic fluid fed from a high-pressure fluid supply source.
2. Description of Related Art
As one type of brake control apparatus for vehicles, such as automobiles, a so-called brake-by-wire type brake control apparatus is known in the art. An example of the brake-by-wire type brake control apparatus is disclosed in Japanese Patent Application Laid-Open No. 2000-247219. The brake control apparatus includes a master cylinder, a high-pressure fluid supply source, wheel cylinders corresponding to vehicle wheels, and cutoff devices for cutting off communication between the master cylinder and the wheel cylinders when the apparatus is in a brake pressure control mode. The brake control apparatus further includes pressure regulating devices for regulating pressures of a hydraulic fluid supplied from the high-pressure fluid supply source to the respective wheel cylinders, communication control devices for controlling communication between two wheel cylinders on the downstream side of the pressure regulating devices, and a controller for controlling the pressure regulating devices according to the amount of a braking operation performed by a driver.
In the above-mentioned brake control apparatus, even when an abnormality occurs in one of the pressure regulating devices corresponding to one of two wheels associated with two wheel cylinders that may communicate with each other via a corresponding communication control device, the communication control device is placed in an operating state for connecting the two wheel cylinders with each other to permit fluid communication, so that the pressure in the wheel cylinder of the above-indicated one wheel can be controlled by controlling the pressure in the wheel cylinder of the other wheel by that other wheel""s pressure regulating device. Therefore, upon occurrence of an abnormality in the pressure regulating device of one wheel, the braking control performance of the braking control apparatus is not reduced or deteriorated very much as compared with the case of a conventional brake-by-wire type brake control apparatus that is typically not provided with communication control devices.
The above-mentioned brake control apparatus may suffer from the following problem. When the controller itself becomes unable to operate normally due to, for example, a reduction in the voltage of a power source for controlling the pressure regulating devices, the pressure regulating devices cannot be controlled normally. In such a case, the brake control apparatus needs to be returned to a non-control mode, by placing the cutoff devices in a position for allowing communication between the master cylinder and the wheel cylinders, so that the pressures in the wheel cylinders are directly controlled by the master cylinder.
It is one object of the invention to provide a brake control apparatus for a vehicle, which is able to continue brake-by-wire type braking force control even when a voltage of a power source for controlling pressure regulating devices is lowered.
To accomplish the above and/or other object(s), there is provided according to one aspect of the invention a brake control apparatus for a motor vehicle, which includes: (a) first and second pressure regulators that regulate a pressure of a hydraulic fluid supplied from a high-pressure fluid supply source to first and second wheel cylinders, (b) a communication control device that controls fluid communication between the first and second wheel cylinders, the communication control device being connected between the first pressure regulator and the first wheel cylinder and between the second pressure regulator and the second wheel cylinder, and (c) a controller that controls the first and second pressure regulators according to at least an amount of a braking operation performed by a vehicle operator. The controller includes first and second control units that control the first and second pressure regulators, respectively, and is operable to determine whether one of the first and second control units may be failing to operate normally. In this brake control apparatus, the communication control device is placed in an operating position for communicating the first and second wheel cylinders with each other when it is determined that the first or second control unit may be failing to operate normally. In this manner, the pressures in the first and second wheel cylinders are controlled to the same level, and are surely prevented from differing from each other to a large extent.
In one embodiment of the invention, when it is determined that one of the first and second control units may be failing to operate normally, the controller stops using the failing one of the first and second control units to control its corresponding pressure regulator, and instead controls both of the first and second pressure regulators using the other control unit (i.e., the control unit that is not failing). Thus, even in the case where one of the first and second control units is not able to continue controlling its corresponding pressure regulating device, the pressures in the first and second wheel cylinders can be surely controlled to the same level by means of the other control unit.
The first and second wheel cylinders as indicated above may be provided for right and left wheels, respectively. In this case, when one of the first and second control units becomes unable to operate normally, the braking forces applied to the right and left wheels will not differ from each other to a large extent, and an excessively large yaw moment is prevented from acting on the vehicle due to the otherwise possible difference between the braking forces for the right and left wheels.
In a further embodiment of the invention, the first control unit includes a first power source that supplies power for controlling the first pressure regulator, and the second control unit includes a second power source that supplies power for controlling the second pressure regulator. In this case, it is determined that the first or second control unit may be failing to operate normally when a voltage of the first or second power source is reduced to be less than a reference value. Thus, when the first or second control unit may become unable to operate normally due to a reduction in the voltage of the first or second power source, the communication control device is reliably operated to communicate the first and second wheel cylinders with each other.
In a yet another embodiment of the invention, when the controller determines that one of the control units may be failing due to a reduction in the voltage of one of the first and second power sources, one of the first and second control units that corresponds to the one of the first and second power sources stops controlling its corresponding pressure regulator, and the other of the first and second control units that corresponds to the other power source controls that pressure regulator. Since the other control unit corresponding to the normally operating power source controls both pressure regulators while the first and second wheel cylinders are held in communication with each other via the communication control device, the pressures in the first and second wheel cylinders can be controlled to the same level with high reliability even if one of the first and second control units is unable to operate normally due to a reduction in the voltage of the corresponding power source.